James Bailey

Mark how long does it take your car to pull from 100 to 140 on the road ??? how long on a roller dyno ?? Huge difference in the time under load and that is the big difference. The roller spins up with no aero drag yet at higher speeds the aero drag is the primary factor. So dyno jet shows what the car should do in an airless environment rolls thru the RPM so quickly you can not even get accurate mixture readings. But yes better than nothing and changes mean you have problems. But it has little to do with how a two cars perform in the real world. Or if pushed under load for a longer time what may happen to the engine in terms of detonation or heat soak....

mark kibort

Those are all reasonable guesses, at best, and you know this. since it seems you are asking the question, ill post an answer.

the dyno takes from 3000rpm to 6000rpm, about 7-8 seconds. that's 4th gear. you can do 3rd gear and get a very exact reading as well, and that takes about 5 seconds. Oh, how coincidental is that??????????????? if I put my car at 60mph in 3rd on the street, I'm at near 3000 rpm and if I punch it and go to redline, just over 100mph, it takes.....................yes, you got it..... about 5 seoonds...............And if you don't have 370rwhp, and are more stock, it takes more like 7 seconds.................hmmmm almost the exact same time as the dyno.................. imagine that Jim!

so, your concern could be a valid one, especaily when it comes to dynos run with cars in two low of gears, but the exhaust gas sensing is very accurate when taking from the cat and even the tail pipe, AND the time we are taking the reading from, is almost identical to actual driving as far as time duration.

Now, lets address your concerns of heat soaking. that is a variable, so you need to be smart about warm up, and cooling fans in front of the car so you get a similar air flow and heat soacking pattern. but multiple runs can cook the engine , but air flow temp is usually a good indication as it responds first. oil tends to lag behind. this is just being smart at the dyno.

so, you are completely off base on your analysis and concern.
Drag is not the major factor either. the rate of acceleration of the mass is.
want proof of this??? any 100hp car can go 120mph.
power= force x speed.
the force to achieve any speed is the aero drag, which is a small percentage of the power required to accelerate. we are talking about acceleration. Now, a brake dyno would be excellent to determine top speed. calculate the force to achieve, say 200mph, set the brake to apply this force at the RPM needed and see if the car can do it.

the rpm also doesn't roll through the test too quickly as I have shown for accurate mixture control. wrong there too.

and if you wanted to simulate longer durations, you can do that too. just do multiple tests with acceleration , brake, acceleration until you reach the limit of the cooling system and dyno fans to check heat soak power.

the dynos are set up to simulate the general time of most cars, with aero drag, proven by the point that most cars will accelerate the load over a period of 7- 9 seconds , which matches real life acceleration.
mixtures are measured well at this time duration, and power is exactly what the dyno says it is! the difference in real life is that ram air effect and cooler air source will actually enhance real world performance. that's one of the reasons my car does better against cars of similar HP. my base of windshield intake with front tubes gets some pretty clean ambient air with out any engine heating, along with higher potential pressure . however, ive already gone over the values of what that pressure will be at 80 vs 160mph, if you recall the discussion many years ago... (i.e. .11psi at 80 and .36psi at 180mph as max potential ram assist)